Process for preparing ethylene oxide catalysts

ABSTRACT

This invention relates to a process for the preparation of an ethylene oxide catalyst having improved selectivity stability which catalyst comprises depositing silver, a promoting amount of alkali metal, a promoting amount of rhenium and, optionally, a promoting amount of rhenium co-promoter selected from sulfur, molybdenum, tungsten, chromium and mixtures thereof, on a porous refractory support, at least partially drying the support, and thereafter depositing a promoting amount of nickel on the support.

FIELD OF THE INVENTION

The invention relates to a process for the preparation ofsilver-containing catalysts suitable for the preparation of ethyleneoxide and to the use of the catalyst for the preparation of ethyleneoxide.

BACKGROUND OF THE INVENTION

Catalysts for the production of ethylene oxide from ethylene andmolecular oxygen are generally supported silver catalysts. Suchcatalysts are typically promoted with alkali metals. The use of smallamounts of the alkali metals potassium, rubidium and cesium were notedas useful promoters in supported silver catalysts in U.S. Pat. No.3,962,136, issued Jun. 8, 1976, and U.S. Pat. No. 4,010,115, issued Mar.1, 1977. The use of other co-promoters, such as rhenium, or rheniumalong with sulfur, molybdenum, tungsten and chromium is disclosed inU.S. Pat. No. 4,766,105, issued Aug. 23, 1988, and U.S. Pat. No.4,808,738, issued Feb. 28, 1989. U.S. Pat. No. 4,908,343, issued Mar.13, 1990, discloses a supported silver catalyst containing a mixture ofa cesium salt and one or more alkali metal and alkaline earth metalsalts.

U.S. Pat. No. 3,844,981 issued Oct. 29, 1974, U.S. Pat. No. 3,962,285issued Jun. 8, 1976 and British Patent No. 1,325,715, published Aug. 8,1973, disclose the use of silver-rhenium ethylene oxide catalysts. Inthese patents a high surface area silver derivative such as silver oxideis impregnated with a rhenium solution and subsequently reduced toprovide metallic rhenium alloyed with the silver. The '285 patentdiscloses the use of KOH to precipitate Ag₂ O from AgNO₃. There is nodisclosure in the patents of the use of suitable inert supports such asporous refractory supports. U.S. Pat. Nos. 4,548,921, issued Oct. 22,1985, discloses the use of rhenium in silver-supported ethylene oxidecatalysts. In this reference, the rhenium is first placed on the supportin the form of finely divided metal particles and the silver issubsequently deposited on the outer surface of the particles. U.S. Pat.No. 3,316,279, issued Apr. 25, 1967, discloses the use of rheniumcompounds, particularly ammonium and alkali metal perrhenates for theoxidation of olefins to olefin oxides. In this reference, however, therhenium compounds are used, unsupported, along with a reaction modifier(cyanides, pyridines or quinolines) in a liquid phase reaction. U.S.Pat. No. 3,972,829, issued Aug. 3, 1976, discloses a method fordistributing catalytically active metallic components on supports usingan impregnating solution of catalyst precursor compound and an organicthioacid or a mercaptocarboxylic acid. Catalytically active metalsinclude metals of Groups IVA, IB, VIB, VIIB and VIII, including rheniumand which may be in either the oxidized or reduced state. U.S. Pat. No.4,459,372, issued Jul. 10, 1984, discloses the use of rhenium metal incombination with a surface metallated (using Ti, Zr, Hf, V, Sb, Pb, Ta,Nb, Ge and/or Si) alumina or silica. U.S. Pat. No. 4,005,049, issuedJan. 25, 1977, teaches the preparation of a silver/transition metalcatalyst useful in oxidation reactions. In this instance, the silverserves as both a catalyst and a support for the transition metalco-catalyst. In U.S. Pat. No. 4,536,482, issued Aug. 20, 1985,catalytically active metals such as Ag and Re are co-sputtered alongwith a co-sputtered support material on a particular support. U.S. Pat.No. 4,257,967, issued Mar. 24, 1981, discloses a catalyst combination ofreduced silver, a carbonate of a rare earth metal and yttrium, a salt ofan alkali or alkaline earth metal and a catalyst carrier. None of thesereferences disclose post-doping a promoting amount of nickel on asilver-based, alkali metal/rhenium-doped supported catalyst.

SUMMARY OF THE INVENTION

The invention relates to a process for the preparation of a catalyst forthe production of ethylene oxide from ethylene and molecular oxygen inthe vapor phase which process comprises depositing a catalyticallyeffective amount of silver, a promoting amount of alkali metal, apromoting amount of rhenium and optionally, a promoting amount of arhenium co-promoter selected from sulfur, molybdenum, tungsten, chromiumand mixtures thereof, on a porous, refractory support, at leastpartially drying the support, depositing a promoting amount of nickel onthe support, and thereafter drying the support.

It has been found that catalysts post-doped or post-impregnated with apromoting amount of nickel have higher selectivity stabilities thanthose obtained with catalysts containing no nickel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, in the vapor phase reaction of ethylene with oxygen toproduce ethylene oxide, the ethylene is present in at least a doubleamount (on a molar basis) compared with oxygen, but frequently is oftenmuch higher. Therefore, the conversion is calculated according to themole percentage of oxygen which has been consumed in the reaction toform ethylene oxide and any oxygenated by-products. The oxygenconversion is dependent on the reaction temperature, and the reactiontemperature is a measure of the activity of the catalyst employed. Thevalue T₄₀ indicates the temperature at 40 mole percent oxygen conversionin the reactor and the value T is expressed in °C. This temperature forany given catalyst is higher when the conversion of oxygen is higher.Moreover, this temperature is strongly dependent on the employedcatalyst and the reaction conditions. The selectivity (to ethyleneoxide) indicates the molar amount of ethylene oxide in the reactionproduct compared with the total molar amount of ethylene converted. Inthis specification, the selectivity is indicated as S₄₀, which means theselectivity at 40 mole percent oxygen conversion. The selectivity ofsilver-based ethylene oxide catalysts can and will decrease over aperiod of time of usage. Therefore, from an economic and practicalstandpoint, it is not only the initial selectivity of a catalyst whichis important, but also the rate at which the selectivity declines. Infact, significant improvement in lowering the decline rate of a catalystcan prove more economically attractive than a high initial selectivity.Thus, the rate at which a catalyst loses selectivity is a predominantfactor influencing the efficiency of any particular catalyst, andlowering this decline rate can lead to significant savings in terms ofminimizing waste of the ethylene starting material. As used herein,"selectivity" is used to refer to the selectivity of ethylene oxidecatalysts when measured at a given constant oxygen conversion level of40% at a gas hourly space velocity of approximately 3300 and whenmeasured after the catalyst has been placed on stream for at leastseveral days.

The catalysts of the instant invention comprise a catalyticallyeffective amount of silver, a promoting amount of alkali metal, apromoting amount of rhenium, a promoting amount of nickel and,optionally, a promoting amount of a rhenium co-promoter selected fromsulfur, chromium, molybdenum, tungsten and mixtures thereof, supportedon a porous, refractory support.

In general, the catalysts of the present invention are prepared byimpregnating porous refractory supports with silver ions or compound(s),complex(es) and/or salt(s) dissolved in a suitable solvent sufficient tocause deposition on the support of from about 1 to about 40 percent byweight, preferably from about 1 to about 25 percent by weight, basis theweight of the total catalyst, of silver. The impregnated support is thenseparated from the solution and the deposited silver compound is reducedto metallic silver. Also deposited on the support either prior to,coincidentally with, or subsequent to the deposition of the silver willbe suitable ions, or compound(s) and/or salt(s) of alkali metaldissolved in a suitable solvent. Also deposited on the support eitherprior to, coincidentally with, or subsequent to the deposition of thesilver and/or alkali metal will be suitable rhenium ions or compound(s),complex(es) and/or salt(s) dissolved in an appropriate solvent. In apreferred embodiment, suitable ions or salt(s), complex(es) and/orcompound(s) of sulfur, molybdenum, tungsten and/or chromium dissolved inan appropriate solvent will be deposited on the carrier either prior to,coincidentally with, or subsequent to the deposition of the silverand/or alkali metal and/or rhenium. Deposited on the carrier followingthe deposition of the silver, alkali metal, rhenium, and rheniumco-promoter, if present, will be suitable nickel compound(s),complex(es) and/or salt(s) dissolved in an appropriate solvent.

The carrier or support employed in these catalysts in its broadestaspects can be any of the large number of conventional, porousrefractory catalyst carriers or support materials which are consideredrelatively inert in the presence of ethylene oxidation feeds, productsand reaction conditions. Such conventional materials are known to thoseskilled in the art and may be of natural or synthetic origin andpreferably are of a macroporous structure, i.e., a structure having asurface area below about 10 m² /g and preferably below about 3 m² /g.Particularly suitable supports are those of aluminous composition.Examples of supports which have been used as supports for differentcatalysts and which could, it is believed, be used as supports forethylene oxide catalysts are the aluminum oxides (including thematerials sold under the trade name "Alundum"), charcoal, pumice,magnesia, zirconia, keiselguhr, fuller's earth, silicon carbide, porousagglomerates comprising silica and/or silicon carbide, silica, magnesia,selected clays, artificial and natural zeolites and ceramics. Refractorysupports especially useful in the preparation of catalysts in accordancewith this invention comprise the aluminous materials, in particularthose comprising alpha alumina. In the case of alpha alumina-containingsupports, preference is given to those having a specific surface area asmeasured by the B.E.T. method of from about 0.03 m² /g to about 10 m²/g, preferably from about 0.05 m² /g to about 5 m² /g, more preferablyfrom about 0.1 m² /g to about 3 m² /g, and a water pore volume asmeasured by conventional water absorption techniques of from about 0.1to about 0.75 cc/g by volume. The B.E.T. method for determining specificsurface area is described in detail in Brunauer, S., Emmet, P. Y. andTeller, E., J. Am. Chem. Soc., 60, 309-16 (1938).

Certain types of alpha alumina containing supports are particularlypreferred. These alpha alumina supports have relatively uniform porediameters and are more fully characterized by having B.E.T. specificsurface areas of from about 0.1 m² /g to about 3 m² /g, preferably fromabout 0.1 m² /g to about 2 m² /g, and water pore volumes of from about0.10 cc/g to about 0.55 cc/g. Typical properties of some supports foundparticularly useful in the present invention are presented in Table I.Suitable manufacturers of carriers comparable to those in Table Iinclude Norton Company and United Catalysts, Inc. (UCI).

                                      TABLE I                                     __________________________________________________________________________    Carrier                    A   B   C    D   E   F                             __________________________________________________________________________    B.E.T. Surface Area, m.sup.2 /g.sup.(a)                                                                  0.21                                                                              0.42                                                                              0.51 0.48                                                                              0.57                                                                              2.06                          Water Pore Volume, cc/g    0.26                                                                              0.36                                                                              0.38 0.49                                                                              0.44                                                                              0.65                          Crush Strength, FPCS, lbs.sup.(b)                                                                        100%                                                                              97% 21   14  15  No Data                                                  20 lbs                                                                            15                                             Total Pore Volume, Hg, cc/g.sup.(c)                                                                      1.26                                                                              0.42                                                                              0.40 0.46                                                                              0.42                                                                              0.65                          Average Pore Diameter, Hg, Å.sup.(c)                                                                 620 560      550 770 1000                          Median Pore Diameter, Hg, microns.sup.(c,d)                                                              3.7 2.7 3.5  3.4 2.4 2.5                           Percent Pore Volume in Pores Greater Than 350Å.sup.(c)                                               90.0%                                                                             88.5%                                                                             93.0%                                                                              89.1%                                                                             91.5%                                                                             94.1%                         Percent Pore Volume in Pores Greater Than 1 Micron.sup.(c)                                               87.0%                                                                             82.5%                                                                             77.0%                                                                              82.3%                                                                             83.5%                                                                             61.0%                         % Wt. Alpha Alumina        99.5                                                                              98  98.8 98.5                                                                              98  70-75                         Water Leachable Na, ppmw   12  53  69   24  18  No Data                       Acid-Leachable Na, ppmw    40  96  188  51  45  No Data                       Water Leachable K, ppmw    5   22  32   22  10  No Data                       Acid-Leachable Fe, ppmw    2   5   No Data                                                                            1   5   No Data                       % Wt. SiO.sub.2            .5  2   0.1  15  2   25-30                         __________________________________________________________________________      .sup.(a) Method of Brunauer, Emmet and Teller, loc. cit.                     .sup.(b) Flat Plate Crush Strength, single pellet.                            .sup.(c) Determined by mercury intrusion to 55,000 psia using Micrometric     Autopore 9200 or 9210 (130° Contact angle, 0.473 N/m surface           tension of Hg).                                                               .sup.(d) Median pore diameter represents the pore diameter wherein 50% of     the total pore volume is found in pores having less than (or greater than     the median pore diameter.                                                

Of the carriers listed in TABLE I, B and C are preferred because theyprovide catalysts which have high initial selectivities.

The support, irrespective of the character of the support or carrierused, is preferably shaped into particles, chunks, pieces, pellets,rings, spheres, wagon wheels, and the like of a size suitable for use infixed bed reactors. Conventional commercial fixed bed reactors aretypically in the form of a plurality of parallel elongated tubes (in asuitable shell) approximately 0.7 to 2.7 inches O.D. and 0.5 to 2.5inches I.D. and 15-45 feet long filled with catalyst. In such reactors,it is desirable to use a support formed into a rounded shape, such as,for example, spheres, pellets, rings, tablets and the like, havingdiameters from about 0.1 inch to about 0.8 inch.

Particular supports having differing properties such as surface area andpore volume may be selected in order to provide particular catalyticproperties. With regard to surface area (B.E.T.), a possible lower limitis about 0.01 m² /g and a possible upper limit is about 10 m² /g. Withregard to water pore volume, a possible lower limit is about 0.05 cc/gand a possible upper limit is about 0.8 cc/g.

The catalysts of the present invention are prepared by a technique inwhich the alkali metal promoters, the rhenium, and the rheniumco-promoter, if present, in the form of soluble salts and/or compoundsare deposited on the catalyst and/or support prior to, simultaneouslywith, or subsequent to the deposition of the silver and each other.Following the deposition of the silver, alkali metal, rhenium andrhenium co-promoter, if present, on the support, the catalyst issubjected to partial drying or to a thermal treatment sufficient toallow deposition of a promoting amount of nickel and thereafter, anickel promoter in the form of soluble salts and/or compounds isdeposited on the catalyst. The alkali metals may be deposited at onestep of the process and the rhenium and/or the rhenium co-promoter, ifpresent, at a different step or steps. The preferred method is todeposit silver, alkali metal, rhenium and rhenium co-promotersimultaneously on the support, that is, in a single impregnation step,although it is believed that the individual or concurrent deposition ofthe alkali metal, rhenium and rhenium co-promoter, if present, prior toand/or subsequent to the deposition of the silver would also producesuitable catalysts. The nickel, however must be deposited on thecatalyst after all of the other catalyst components have been added andafter the catalyst has been at least partially dried to a degreesufficient to permit a promoting amount of nickel to be impregnated ontothe catalyst. The post-impregnation or post-doping of the catalyst witha promoting amount of nickel compounds results in a catalyst havingimproved selectivity stability.

Promoting amounts of alkali metal or mixtures of alkali metal aredeposited on a porous support using a suitable solution. Although alkalimetals exist in a pure metallic state, they are not suitable for use inthat form. They are used as ions or compounds of alkali metals dissolvedin a suitable solvent for impregnation purposes. The carrier isimpregnated with a solution of alkali metal promoter ions, salt(s)and/or compound(s) before, during or after impregnation of the silverions or salt(s), complex(es), and/or compound(s) has taken place. Analkali metal promoter may even be deposited on the carrier afterreduction to metallic silver has taken place. The promoting amount ofalkali metal utilized will depend on several variables, such as, forexample, the surface area and pore structure and surface chemicalproperties of the carrier used, the silver content of the catalyst andthe particular ions used in conjunction with the alkali metal cation,nickel, rhenium and rhenium co-promoter, if present, and the amounts ofnickel, rhenium and rhenium co-promoter, if any, present. The amount ofalkali metal promoter deposited upon the support or present on thecatalyst generally lies between about 10 parts per million and about3000 parts per million, preferably between about 15 parts per millionand about 2000 parts per million and more preferably, between about 20parts per million and about 1500 parts per million by weight of totalcatalyst. Most preferably, the amount ranges between about 50 parts permillion and about 1000 parts per million by weight of the totalcatalyst. The degree of benefit obtained within the above-defined limitswill vary depending upon particular properties and characteristics, suchas, for example, reaction conditions, catalyst preparative techniques,surface area and pore structure and surface chemical properties of thecarrier utilized, silver content of the catalyst, and other compounds,cations or anions present in addition to alkali metal ions such as ionsadded with the alkali metal, nickel, rhenium and rhenium co-promoter, ifpresent, or compounds remaining from the impregnating solution, and theabove-defined limits were selected to cover the widest possiblevariations in properties and characteristics. The effects of thesevariations in properties are readily determined by experimentation. Thealkali metal promoters are present on the catalysts in the form ofcations (ions) or compounds of complexes or surface compounds or surfacecomplexes rather than as the extremely active free alkali metals,although for convenience purposes in this specification and claims theyare referred to as "alkali metal" or "alkali metal promoters" eventhough they are not present on the catalyst as metallic elements. Forpurposes of convenience, the amount of alkali metal deposited on thesupport or present on the catalyst is expressed as the metal. Withoutintending to limit the scope of the invention, it is believed that thealkali metal compounds are oxidic compounds. More particularly, it isbelieved that the alkali metal compounds are probably in the form ofmixed surface oxides or double surface oxides or complex surface oxideswith the aluminum of the support and/or the silver of the catalyst,possibly in combination with species contained in or formed from thereaction mixture, such as, for example, chlorides or carbonates orresidual species from the impregnating solution(s).

In a preferred embodiment, at least a major proportion (greater than50%) of the alkali metals comprise the higher alkali metals. As usedherein, the term "higher alkali metal" and cognates thereof refers tothe alkali metals selected from the group consisting of potassium,rubidium, cesium and mixtures thereof. As used herein, the term "alkalimetal" and cognates thereof refers to the alkali metals selected fromthe group consisting of lithium, sodium, potassium, rubidium, cesium andmixtures thereof. As used herein, the term "mixtures of alkali metals"or "mixtures of higher alkali metals" or cognates of these terms refersto the use of two or more of the alkali or higher alkali metals, asappropriate, to provide a promoting effect. Non-limiting examplesinclude cesium plus rubidium, cesium plus potassium, cesium plus sodium,cesium plus lithium, cesium plus rubidium plus sodium, cesium pluspotassium plus sodium, cesium plus lithium plus sodium, cesium plusrubidium plus potassium plus sodium, cesium plus rubidium plus potassiumplus lithium, cesium plus potassium plus lithium and the like. Apreferred alkali metal promoter is cesium. A particularly preferredalkali metal promoter is cesium plus at least one additional alkalimetal. The additional alkali metal is preferably selected from sodium,lithium and mixtures thereof, with lithium being preferred.

It should be understood that the amounts of alkali metal promoters onthe catalysts are not necessarily the total amounts of these metalspresent in the catalyst. Rather, they are the amounts of alkali metalpromoters which have been added to the catalyst by impregnation with asuitable solution of ions, salts and/or compounds and/or complexes ofalkali metals. These amounts do not include amounts of alkali metalswhich are locked into the support, for example, by calcining, or are notextractable in a suitable solvent such as water or lower alkanol oramine or mixtures thereof and do not provide a promoting effect. It isalso understood that a source of the alkali metal promoter ions, saltsand/or compounds used to promote the catalyst may be the carrier. Thatis, the carrier may contain extractable amounts of alkali metal that canbe extracted with a suitable solvent, such as water or lower alkanol,thus preparing an impregnating solution from which the alkali metalions, salts and/or compounds are deposited or redeposited on thesupport.

As used herein, the term "compound" refers to the combination of aparticular element with one or more different elements by surface and/orchemical bonding, such as ionic and/or covalent and/or coordinatebonding. The term "ionic" or "ion" refers to an electrically chargedchemical moiety; "cationic" or "cation" being positive and "anionic" or"anion" being negative. The term "oxyanionic" or "oxyanion" refers to anegatively charged moiety containing at least one oxygen atom incombination with another element. An oxyanion is thus anoxygen-containing anion. It is understood that ions do not exist invacuo, but are found in combination with charge-balancing counterions.The term "oxidic" refers to a charged or neutral species wherein anelement in question is bound to oxygen and possibly one or moredifferent elements by surface and/or chemical bonding, such as ionicand/or covalent and/or coordinate bonding. Thus, an oxidic compound isan oxygen-containing compound which also may be a mixed, double orcomplex surface oxide. Illustrative oxidic compounds include, by way ofnon-limiting examples, oxides (containing only oxygen as the secondelement), hydroxides, nitrates, sulfates, carboxylates, carbonates,bicarbonates, oxyhalides, etc. as well as surface species wherein theelement in question is bound directly or indirectly to an oxygen eitherin the substrate or the surface.

As used herein, the term "promoting amount" of a certain component of acatalyst refers to an amount of that component which works effectivelyto provide an improvement in one or more of the catalytic properties ofthat catalyst when compared to a catalyst not containing said component.Examples of catalytic properties include, inter alia, operability(resistance to runaway), selectivity, activity, conversion, stabilityand yield. It is understood by one skilled in the art that one or moreof the individual catalytic properties may be enhanced by the "promotingamount" while other catalytic properties may or may not be enhanced andmay even be diminished. It is further understood that differentcatalytic properties may be enhanced at different operation conditions.For example, a catalyst having enhanced selectivity at one set ofoperating conditions may be operated at a different set of conditionswherein the improvement shows up in the activity rather than theselectivity and an operator of an ethylene oxide plant willintentionally change the operation conditions in order to take advantageof certain catalytic properties even at the expense of other catalyticproperties in order to maximize profits by taking into account feedstockcosts, energy costs, by-product removal costs and the like. Theparticular combination of silver, support, alkali metal promoter,rhenium promoter, rhenium co-promoter, if present, and post-doped nickelpromoter of the instant invention will provide an improvement in one ormore catalytic properties over the same combination of silver, support,alkali metal promoter, rhenium promoter and rhenium co-promoter, ifpresent, and no post-doped nickel promoter.

As used herein, the term "catalytically effective amount of silver"refers to an amount of silver that provides a measurable conversion ofethylene and oxygen to ethylene oxide.

The carrier is also impregnated with rhenium ions, salt(s), compound(s),and/or complex(es). This may be done at the same time that the alkalimetal promoter is added, or before or later; or at the same time thatthe silver is added, or before or later; or at the same time that therhenium co-promoter, if present, is added, or before or later.Preferably, rhenium, alkali metal, rhenium co-promoter, if present, andsilver are in the same impregnating solution, although it is believedthat their presence in different solutions will still provide suitablecatalysts. The preferred amount of rhenium, calculated as the metal,deposited on or present on the carrier or catalyst ranges from about 0.1micromoles per gram to about 10 micromoles per gram, more preferablyfrom about 0.2 micromoles per gram to about 5 micromoles per gram oftotal catalyst, or, alternatively stated, from about 19 parts permillion to about 1860 parts per million, preferably from about 37 partsper million to about 930 parts per million by weight of total catalyst.The degree of benefit obtained within the above-defined limits will varydepending upon particular properties and characteristics, such as, forexample, reaction conditions, catalyst preparative conditions, surfacearea and pore structure and surface chemical properties of the carrierutilized, silver content of the catalyst and other compounds, anions orcations present in addition to those containing rhenium, alkali metal,or rhenium co-promoter, such as ions added with the alkali metal,rhenium or rhenium co-promoter, or compounds remaining from theimpregnating technique, and the above-defined limits were selected tocover the widest possible variations in properties and characteristics.The effects of these variations are readily determined byexperimentation. For purposes of convenience, the amount of rheniumpresent on the catalyst is expressed as the metal, irrespective of theform in which it is present.

The rhenium compounds used in the preparation of the instant catalystsare rhenium compounds that can be solubilized in an appropriate solvent.Preferably, the solvent is a water-containing solvent. More preferably,the solvent is the same solvent used to deposit the silver and thealkali metal promoter. Examples of suitable rhenium compounds includethe rhenium salts such as rhenium halides, the rhenium oxyhalides, therhenates, the perrhenates, the oxides and the acids of rhenium. Apreferred compound for use in the impregnation solution is theperrhenate, preferably ammonium perrhenate. However, the alkali metalperrhenates, alkaline earth metal perrhenates, silver perrhenates, otherperrhenates and rhenium heptoxide can also be suitably utilized. Rheniumheptoxide, Re₂ O₇, when dissolved in water, hydrolyzes to perrhenicacid, HReO₄, or hydrogen perrhenate. Thus, for purposes of thisspecification, rhenium heptoxide can be considered to be a perrhenate,i.e., ReO₄. It is also understood that there are many rhenium compoundsthat are not soluble per se in water. However, these compounds can besolubilized by utilizing various acids, bases, peroxides, alcohols, andthe like. After solubilization, these compounds could be used, forexample, with an appropriate amount of water or other suitable solventto impregnate the carriers. Of course, it is also understood that uponsolubilization of many of these compounds, the original compound nolonger exists after solubilization. For example, rhenium metal is notsoluble in water. However, it is soluble in concentrated nitric acid aswell as in hydrogen peroxide solution. Thus, by using an appropriatereactive solvent, one could use rhenium metal to prepare a solubilizedrhenium-containing impregnating solution. In a preferred embodiment ofthe instant invention, the rhenium present on the catalyst is present ina form that is extractable in a dilute aqueous base solution.

It was found in U.S. Pat. No. 4,766,105, issued Aug. 23, 1988, that if arhenium co-promoter is added to an alkali metal/rhenium doped supportedsilver catalyst, an improvement in initial selectivity is obtained.While suitable catalysts can be prepared in the absence of a rheniumco-promoter, it is preferable that the catalyst in the present inventioncontain a rhenium co-promoter. When a co-promoter is utilized, theco-promoter is selected from the group consisting of sulfur, molybdenum,tungsten, chromium and mixtures thereof, preferably a compound ofsulfur, molybdenum, tungsten, chromium and mixtures thereof. The exactform of the co-promoter on the catalyst is not known. The co-promoter,it is believed, is not present on the catalyst in the elemental formsince the co-promoter is applied to catalyst in the form of ions, salts,compounds and/or complexes and the reducing conditions generally used toreduce the silver to metallic silver are not usually sufficient toreduce the sulfur, molybdenum, tungsten or chromium to the elementalform. It is believed that the co-promoter deposited on the support orpresent on the catalyst is in the compound form, and probably in theform of an oxygen-containing or oxidic compound. In a presentlypreferred embodiment, the co-promoter is applied to the catalyst in theoxyanionic form, i.e, in the form of an anion, or negative ion whichcontains oxygen. Examples of anions of sulfur that can be suitablyapplied include sulfate, sulfite, bisulfite, bisulfate, sulfonate,persulfate, thiosulfate, dithionate, etc. Preferred compounds to beapplied are ammonium sulfate and the alkali metal sulfates. Examples ofanions of molybdenum, tungsten and chromium that can be suitably appliedinclude molybdate, dimolybdate, paramolybdate, other iso- andhetero-polymolybdates, etc.; tungstate, paratungstate, metatungstate,other iso- and hetero-polytungstates, etc.; and chromate, dichromate,chromite, halochromate, etc. Preferred are sulfates, molybdates,tungstates and chromates. The anions can be supplied with variouscounter-ions. Preferred are ammonium, alkali metal and hydrogen (i.e.acid form). The anions can be prepared by the reactive dissolution ofvarious non-anionic materials such as the oxides such as SO₂, SO₃, MoO₃,WO₃, Cr₂ O₃, etc., as well as other materials such as halides,oxyhalides, hydroxyhalides, hydroxides, sulfides, etc., of the metals.

When a co-promoter is used, the carrier is impregnated with rheniumco-promoter ions, salt(s), compound(s) and/or complex(es). This may bedone at the same time that the other components are added, or beforeand/or later. Preferably, rhenium co-promoter, rhenium, alkali metal andsilver are in the same impregnating solution, although it is believedthat their presence in different solutions will still provide suitablecatalysts.

The preferred amount of co-promoter compound present on or deposited onthe support or catalyst ranges from about 0.1 to about 10 micromoles,preferably from about 0.2 to about 5 micromoles, expressed as theelement, per gram of total catalyst. The degree of benefit obtainedwithin the above-defined limits will vary depending upon particularproperties and characteristics, such as, for example, reactionconditions, catalyst preparative techniques, surface area and porestructure and surface chemical properties of the carrier utilized,silver content of the catalyst, alkali content of the catalyst, rheniumcontent of the catalyst, and other compounds anions or cations presentbesides those containing rhenium, rhenium co-promoter and alkali metal,such as ions added with the alkali metal, rhenium or rheniumco-promoter, or compounds remaining from the impregnation technique, andthe above-defined limits were selected to cover the widest possiblevariations in properties and characteristics. These variations arereadily determined by experimentation. For purposes of convenience theamount of co-promoter present on the catalyst is expressed as theelement irrespective of the form in which it is present.

The presence of the indicated and claimed promoting amount of rheniumco-promoters in this specification and claims does not preclude the useof other activators, promoters, enhancers, stabilizers, improvers, etc.,and it is not intended by the use of Markush terminology in thisspecification and claims to exclude the use of such other activators,promoters, enhancers, stabilizers, improvers, etc.

The co-promoter compounds, salts and/or complexes suitable for use inthe preparation of the instant catalysts are compounds, salts and/orcomplexes which can be solubilized in an appropriate solvent.Preferably, the solvent is a water-containing solvent. More preferably,the solvent is the same solvent used to deposit the silver, alkali metalpromoter and rhenium. Preferred co-promoter compounds are the oxyanioniccompounds of the co-promoter elements, preferably the ammonium andalkali metal oxyanionates, such as ammonium sulfate, potassium sulfate,cesium chromate, rubidium tungstate, ammonium molybdate, lithiumsulfate, sodium tungstate, lithium chromate and the like.

Following the impregnation of the carrier with silver, alkali metal,rhenium and rhenium co-promoter, if present, the impregnated carrier isat least partially dried to a degree sufficient to allow forimpregnation of the carrier with nickel ions, salt(s), compound(s)and/or complex(es). It is necessary for purposes of this invention thatthe nickel be added after the other components, i.e., the silver, alkalimetal, rhenium and rhenium co-promoter, if present, have beenimpregnated or deposited on the support and the impregnated support orcatalyst has been at least partially dried to an extent sufficient toallow an essentially non-aqueous solution of nickel ions to be adsorbedinto the pore structure of the support or catalyst. The solution ofnickel ions must be essentially non-aqueous in order to prevent thepotential redissolution and redistribution of the other components,i.e., the silver, alkali metal, rhenium and rhenium co-promoter, ifpresent, which have already been deposited on the support. Althoughnickel does exist in a pure metallic state, it is not suitable for usein that form. The nickel is used as an ion or compound of nickeldissolved in a suitable solvent for impregnation purposes. The catalystis post-impregnated or post-doped with a solution of nickel promoterions, salt(s) and/or compound(s) after impregnation of the silver ionsor salt(s), complex(es), and/or compound(s) and the other promoters,i.e., alkali metal, rhenium, and optionally, rhenium co-promoter, hastaken place. As used herein, the terms "post-doping" and"post-impregnation" and similar terms are used to refer to the additionof a nickel promoter to a catalyst or support on which silver, alkalimetal, rhenium and rhenium co-promoter, if any, have already beendeposited or impregnated, and which has been at least partially dried.The promoting amount of nickel utilized will depend on severalvariables, such as, for example, the surface area and pore structure andsurface chemical properties of the carrier used, the silver content ofthe catalyst and the particular ions used in conjunction with the alkalimetal cation, rhenium or rhenium co-promoter, if present, and amounts ofalkali metal, rhenium and rhenium co-promoter, if any, present. Theamount of nickel promoter deposited upon the support or present on thecatalyst generally lies between about 10 parts per million and about1000 parts per million, and preferably between about 15 parts permillion and about 500 parts per million by weight of the total catalyst.Most preferably, the amount ranges between about 30 parts per millionand about 250 parts per million by weight of the total catalyst. Thepromoting effect provided by the nickel will vary depending uponparticular properties and characteristics, such as, for example,reaction conditions, catalyst preparative techniques, surface area andpore structure and surface chemical properties of the carrier utilized,the silver, alkali metal, rhenium and rhenium co-promoter content of thecatalyst, and other compounds, cations or anions present in addition tothose containing the alkali metal, rhenium and rhenium co-promoter, ifpresent, or compounds remaining from the impregnating solution, and theabove-defined limits were selected to cover the widest possiblevariations in properties and characteristics. The effect, of thesevariations in properties are readily determined by experimentation. Thenickel promoter or promoters are presumably present on the catalyst inthe form of oxides or oxygen-bound species, or surface compounds orsurface complexes rather than as metals, although for purposes ofconvenience in this specification and claims, they are referred to as"nickel", "nickel compound(s)", and/or "nickel promoter(s)" even thoughthey are not present on the catalyst as metals. For purposes ofconvenience, the amount of nickel deposited on the support or present onthe catalyst is expressed as the element rather than in the cationic orcompounds or complexes or surface compounds or surface complexes.Without intending to limit the scope of the invention, it is believedthat the nickel compounds are oxidic compounds. More particularly, it isbelieved that the nickel compounds are probably in the form of mixedsurface oxides or double surface oxides or complex surface oxides withthe aluminum of the support and/or the silver of the catalyst, possiblyin combination with species contained in or formed from the reactionmixture, such as, for example, chlorides or carbonates or residualspecies from the impregnating solution(s).

As used herein, the term "mixtures of nickel compounds" refers to theuse of two or more compounds of nickel, as appropriate, to provide apromoting effect. In a preferred embodiment, the nickel compound isselected from the group consisting of nickel carbonate, nickel nitrate,nickel acetate, nickel acetylacetonate, nickel chloride, nickel sulfateand mixtures thereof, with nickel carbonate, nickel nitrate, nickelsulfate and mixtures thereof being particularly preferred. Particularlypreferred nickel promoters are nickel carbonate and nickel nitrate.

The promoting effect provided by the nickel can be affected by a numberof variables such as for example, reaction conditions, catalystpreparative techniques, surface area and pore structure and surfacechemical properties of the support, the silver, alkali metal, rheniumand, if present, rhenium co-promoter content of the catalyst, and thepresence of other cations and anions present on the catalyst alone or incombination with the alkali metal and/or rhenium and/or rheniumco-promoter and/or nickel. The presence of other activators,stabilizers, promoters, enhancers or other catalyst improvers can alsoaffect the promoting effects of the nickel. It is understood that anysupported silver-based, alkali metal/rhenium promoted ethylene oxidecatalyst which contains other cations and/or anions or any otheractivators, promoters, enhancers, stabilizers or the catalyst improversand which contains a post-doped amount of nickel which provides apromoting effect, more preferably which provides higher selectivitystabilities than those obtained under the same reaction conditions withthe same catalyst which contains no post-doped nickel promoter, willfall within the scope of the instant invention and claims.

Generally, the carrier is contacted with a silver salt, a silvercompound, or a silver complex which has been dissolved in an aqueoussolution, so that the carrier is impregnated with said aqueous solution;thereafter the impregnated carrier is separated form the aqueoussolution, e.g., by centrifugation or filtration and then dried. The thusobtained impregnated carrier is heated to reduce the silver to metallicsilver. It is understood that the reduction of the silver to metallicsilver may take place, at least in part, during the partial drying stepwhich takes place before the deposition of the nickel promoter. It isconveniently heated to a temperature in the range of from about 50° C.to about 600° C., during a period sufficient to cause reduction of thesilver salt, compound or complex to metallic silver and to form a layerof finely divided silver, which is bound to the surface of the carrier,both the exterior and pore surface. Air, or other oxidizing gas,reducing gas, an inert gas or mixtures thereof may be conducted over thecarrier during this heating step.

There are several known methods to add the silver to the carrier orsupport. The carrier may be impregnated with an aqueous solutioncontaining silver nitrate dissolved therein, and then dried, after whichdrying step the silver nitrate is reduced with hydrogen or hydrazine.The carrier may also be impregnated with an ammoniacal solution ofsilver oxalate or silver carbonate, and then dried, after which dryingstep the silver oxalate or silver carbonate is reduced to metallicsilver by heating, e.g., to about 600° C. Specific solutions of silversalts with solubilizing and reducing agents may be employed as well,e.g., combinations of the vicinal alkanolamines, alkyldiamines andammonia. One such example of a solution of silver salts comprises animpregnating solution comprising a silver salt of a carboxylic acid, anorganic amine alkaline solubilizing/reducing agent, and an aqueoussolvent.

Suitable carboxylic acid silver salts include silver carbonate and thesilver salts of mono- and polybasic carboxylic and hydroxycarboxylicacids of up to about 16 carbon atoms. Silver carbonate and silveroxalate are particularly useful silver salts, with silver oxalate beingmost preferred.

An organic amine solubilizing/reducing agent is present in theimpregnating solution. Suitable organic aminesilver-solubilizing/reducing agents include lower alkylenediamines offrom 1 to 5 carbon atoms, mixtures of a lower alkanolamine of from i to5 carbon atoms with a lower alkylenediamine of from 1 to 5 carbon atoms,as well as mixtures of ammonia with lower alkanolamines or loweralkylenediamines for from 1 to 5 carbons. Four groups of organic aminesolubilizing/reducing agents are particularly useful. The four groupsinclude vicinal alkylenediamines of from 2 to 4 carbon atoms, mixturesof (1) vicinal alkanolamines of from 2 to 4 carbon atoms and (2) vicinalalkylenediamines of from 2 to 4 carbon atoms, mixtures of vicinalalkylenediamines of from 2 to 4 carbon atoms and ammonia, and mixturesof vicinal alkanolamines of from 2 to 4 carbon atoms and ammonia. Thesesolubilizing/reducing agents are generally added in the amount of fromabout 0.1 to about 10 moles per mole of silver present.

Particularly preferred solubilizing/reducing agents are ethylenediamine,ethylenediamine in combination with ethanolamine, ethylenediamine incombination with ammonia, and ethanolamine in combination with ammonia,with ethylenediamine being most preferred. Ethylenediamine incombination with ethanolamine gives comparable results, but it isbelieved that impurities present in certain commercially availableethanolamine preparations can produce inconsistent results.

When ethylenediamine is used as the sole solubilizing/reducing agent, itis necessary to add amount of the amine in the range of from about 0.1to about 5.0 moles of ethylenediamine per mole of silver.

When ethylenediamine and ethanolamine together are used as thesolubilizing/reducing agent, it is suitable to employ from about 0.1 toabout 3.0 moles of ethylenediamine per mole of silver and from about 0.1to about 2.0 moles of ethanolamine per mole of silver.

When ethylenediamine or ethanolamine is used with ammonia, it isgenerally useful to add at least about two moles of ammonia per mole ofsilver and very suitable to add from about 2 to about 10 moles ofammonia per mole of silver. The amount of ethylenediamine orethanolamine employed then is suitably from 0.1 to 2.0 moles per mole ofsilver.

One method of preparing the silver containing catalyst can be found inU.S. Pat. No. 3,702,259, issued Nov. 7, 1972, incorporated by referenceherein. Other methods for preparing the silver-containing catalystswhich in addition contain higher alkali metal promoters can be found inU.S. Pat. No. 4,010,115, issued Mar. 1, 1977; and U.S. Pat. No.4,356,312, issued Oct. 26, 1982; U.S. Pat. No. 3,962,136, issued Jun. 8,1976 and U.S. Pat. No. 4,012,425, issued Mar. 15, 1977, all incorporatedby reference herein. Methods for preparing silver-containing catalystscontaining higher alkali metal and rhenium promoters can be found inU.S. Pat. No. 4,761,394, issued Aug. 2, 1988, which is incorporated byreference herein, and methods for silver-containing catalysts containinghigher alkali metal and rhenium promoters and a rhenium co-promoters canbe found in U.S. Pat. No. 4,766,105, issued Aug. 2, 1988, which isincorporated herein be reference.

The preferred amount of alkali metal promoter deposited on or present onthe surface of the carrier of catalyst generally lies between about 10parts per million and about 3000 parts per million, preferably betweenabout 15 parts per million and about 2000 parts per million and morepreferably between about 20 parts per million and about 1500 parts permillion by weight of alkali metal calculated on the total carriermaterial. Amounts between about 50 parts per million and about 1000parts per million are most preferable. Suitable compounds of alkalimetals comprise lithium, sodium, potassium, rubidium, cesium or mixturesthereof in a promoting amount with the even more preferred promotersbeing rubidium and/or cesium plus an additional alkali metal selectedfrom lithium, sodium and mixtures thereof. Preferably the amount rangesfrom about 10 parts per million and about 3000 parts per million, morepreferably between about 15 parts per million and about 2000 parts permillion, even more preferably between about 20 parts per million andabout 1500 parts per million by weight, and most preferably betweenabout 50 parts per million and 1000 parts per million by weight. Themost preferred promoter is cesium plus lithium, preferably applied in anaqueous solution having cesium nitrate or cesium hydroxide dissolvedtherein.

There are known excellent methods of applying the alkali metal andrhenium promoters, and rhenium co-promoter, if present, coincidentallywith the silver on the carrier. Suitable alkali metal salts aregenerally those which are soluble in the silver-impregnating liquidphase. Besides the above-mentioned compounds may be mentioned thenitrites; the halides, such as fluorides, chlorides, iodides, bromides;oxyhalides; bicarbonates; borates; sulfates; sulfites; bisulfates;acetates; tartrates; lactates and isopropoxides, etc. The use of alkalimetal, rhenium or co-promoter salts which have ions which react with thesilver salt in solution is preferably avoided, e.g. the use of cesiumchloride together with silver nitrate in an aqueous solution, since thensome silver chloride is prematurely precipitated. Here the use of cesiumnitrate is recommended instead of cesium chloride, for example. However,cesium chloride may be used together with a silver salt-amine-complex inaqueous solution, since then the silver chloride is not precipitatedprematurely from the solution.

The alkali metal and rhenium promoters and rhenium co-promoters, ifpresent, may be deposited on the carrier (support) or on the catalyst,depending upon the particular impregnation technique or sequenceutilized. In this specification and claims, the term "on the catalyst"when referring to the deposition or presence of promoters and/orco-promoters refers to the catalyst which comprises the combination ofcarrier (support) and silver. Thus, the promoters, i.e., alkali metal,rhenium and rhenium co-promoter may be found individually or in amixture thereof on the catalyst, on the support or on both the catalystand the support. There may be, for example, alkali metal, rhenium andrhenium co-promoter on the support; alkali metal, rhenium and rheniumco-promoter on the catalyst; alkali metal, and rhenium on the supportand rhenium co-promoter on the catalyst; alkali metal and rheniumco-promoter on the support and rhenium on the catalyst; alkali metal,rhenium and rhenium co-promoter on the support and rhenium and rheniumco-promoter on the catalyst and any of the other possible distributionsof alkali metal, rhenium and/or rhenium co-promoter between the supportand/or the catalyst.

The nickel promoter(s) is deposited on the catalyst, i.e., thecombination of support and silver, after the deposition of the alkalimetal and rhenium promoters and rhenium co-promoter, if any, and afterthe impregnated catalyst has been at least partially dried. As usedherein, the terms "partially dried" and "partially drying" refer tothermal treatment at a temperature sufficient to remove the solvent fromthe wet precursor. The partial drying temperatures will typically be inthe range of from about 50° C. and about 600° C., preferably betweenabout 75° C. and about 400° C. While not wishing to be bound by anytheory, it is believed that the nickel is present in an ionic form,binding to the perrhenate oxyanions.

The amount of the alkali metal and/or rhenium and/or nickel promotersand/or rhenium co-promoters on the porous carrier or catalyst may alsobe regulated within certain limits by washing out the surplus ofpromoter material with an appropriate solvent, for example, methanol orethanol.

A particularly preferred process of impregnating the carrier withsilver, alkali metal, rhenium and rhenium co-promoter consists ofimpregnating the carrier with an aqueous solution containing a silversalt of a carboxylic acid, an organic amine, a salt of cesium, ammoniumperrhenate and ammonium sulfate dissolved therein. Silver oxalate is apreferred salt. It can be prepared by reacting silver oxide (slurry inwater) with (a) a mixture of ethylenediamine and oxalic acid, or (b)oxalic acid and then ethylenediamine, which latter is preferred, so thatan aqueous solution of silver oxalate-ethylenediamine complex isobtained, to which solution is added a certain amount of cesiumcompound, ammonium perrhenate and ammonium sulfate. While addition ofthe amine to the silver oxide before adding the oxalic acid is possible,it is not preferred since it can give rise to solutions which areunstable or even explosive in nature. Other diamines and other amines,such as ethanolamine, may be added as well. A cesium-containing silveroxalate solution may also be prepared by precipitating silver oxalatefrom a solution of cesium oxalate and silver nitrate and rinsing withwater or alcohol the obtained silver oxalate in order to remove theadhering cesium salt until the desired cesium content is obtained. Thecesium-containing silver oxalate is then solubilized with ammonia and/oran amine in water and ammonium perrhenate and ammonium sulfate is added.Rubidium-, potassium-, sodium-, lithium- and mixtures of alkalimetal-containing solutions may be prepared also in these ways. Theimpregnated carriers are then partially dried at a temperature betweenabout 50° C. and about 600° C., preferably between about 75° C. andabout 400° C. to evaporate the liquid. It is believed that these partialdrying temperatures also result in substantial reduction of the silverto produce a metallic silver. Thereafter, the impregnated carriers arepost-impregnated or post-doped with an essentially alcoholic solutioncontaining nickel cations. The solution can be prepared by dissolving asuitable soluble nickel salt in a substantially non-aqueous solvent suchas, for example, a lower alcohol solvent. The post-doped orpost-impregnated carriers are then heated to a temperature between about50° C. and about 600° C., preferably between about 75° C. and about 400°C. to evaporate the liquid and produce a metallic silver.

In general terms, the impregnation process comprises impregnating thesupport with one or more solutions comprising silver, alkali metal,rhenium and rhenium co-promoter, partially drying the impregnatedsupport, post-impregnating the support with one or more solutionscomprising nickel, and drying the post-impregnated support. As used inthe instant specification and claims, the terminology "impregnating thesupport with one or more solutions comprising silver, alkali metal,rhenium and/or rhenium co-promoter", and similar or cognate terminologymeans that the support is impregnated in a single or multipleimpregnation with one solution containing silver, alkali metal, rheniumand rhenium co-promoter in differing amounts; or in multipleimpregnations with two or more solutions, wherein each solution containsat least one component selected from silver, alkali metal, rhenium andrhenium co-promoter, with the proviso that all of the components ofsilver, alkali metal, rhenium and rhenium co-promoter will individuallybe found in at least one of the solutions. The concentration of thesilver (expressed as the metal) in the silver-containing solution willrange from about 1 g/l up to the solubility limit when a singleimpregnation is utilized. The concentration of the alkali metal(expressed as the metal) will range from about 1×10⁻³ g/l up to about 12g/l and preferably, from about 10×10⁻³ g/l to about 12 g/l when a singleimpregnation step is utilized. The concentration of the rhenium(expressed as the metal) will range from about 5×10⁻³ g/l to about 20g/l and preferably from about 50×10⁻³ g/l to about 20 g/l when a singleimpregnation step is utilized. The concentration of rhenium co-promoter(expressed as the element) will range from about 1×10⁻³ g/l to about 20g/l and preferably from about 10×10⁻³ g/l to about 20 g/l when a singleimpregnation step is utilized. For purposes of the post-impregnationstep, the concentration of the nickel (expressed as the element) willrange from about 1×10⁻⁵ g/l up to about 1 g/l and preferably, from about5×10⁻⁴ g/l to about 0.1 g/l when a single post-impregnation step isutilized. Concentrations selected within the above noted ranges willdepend upon the pore volume of the catalyst, the final amount desired inthe final catalyst and whether the impregnation is single or multiple.Appropriate concentrations can be readily determined by routineexperimentation.

The amount of silver deposited on the support or present on the supportis to be a catalytically effective amount of silver, i.e., an amountthat catalyzes the reaction of ethylene and oxygen to produce ethyleneoxide. Preferably this amount will range from about 1 to about 40percent by weight of the total catalyst, more preferably from about 1 toabout 25 percent by weight of the total catalyst, and even morepreferably from about 5 to about 20 percent by weight of the totalcatalyst. The upper and lower limits of preferred silver concentrationscan be suitably varied, depending upon the particular catalyticproperties or effect desired or the other variables involved. The lowerlimit of silver is probably about 1 percent by weight of the totalcatalyst, and the upper limit of silver is probably about 40 percent byweight of the total catalyst.

The amount of alkali metal deposited on the support or catalyst orpresent on the support or catalyst is to be a promoting amount.Preferably the amount will range from about 10 parts per million toabout 3000 parts per million, more preferably from about 15 parts permillion to about 2000 parts per million and even more preferably fromabout 20 parts per million to about 1500 parts per million, and mostpreferably, from about 50 parts per million to about 1000 parts permillion by weight of the total catalyst, expressed as the metal. Theupper and lower limits of preferred alkali metal concentrations can besuitably varied depending upon the particular promoting effect desiredor other variables involved. A possible lower limit of alkali metal is,for example, about 1 parts per million by weight of the total catalyst,expressed as the metal, and a possible upper limit of alkali metal is,for example, about 3000 parts per million by weight of the totalcatalyst, expressed as the metal.

The amount of rhenium deposited on the support or catalyst or present onthe support of catalyst is to be a promoting amount. Preferably, theamount will range from about 0.1 micromoles per gram to about 10micromoles per gram, preferably from about 0.2 micromoles per gram toabout 5 micromoles per gram, and more preferably, from about 0.5micromoles per gram to about 4 micromoles per gram of total catalyst,expressed as the metal. The upper and lower limits of preferred rheniumconcentrations can be suitably varied depending upon the particularpromoting effect desired or other variables involved. A possible lowerlimit of rhenium is, for example, about 0.01 micromoles per gram oftotal catalyst, and a possible upper limit of rhenium is, for example,about 16 micromoles per gram of total catalyst, expressed as the metal.

The amount of rhenium co-promoter deposited on the support or catalystor present on the support or catalyst is to be a promoting amount.Preferably, the amount will range from about 0.1 micromoles per gram toabout 10 micromoles per gram, preferably from about 0.2 micromoles pergram to about 5 micromoles per gram, and more preferably, from about 0.5micromoles per gram to about 4 micromoles per gram of total catalyst,expressed as the element or the metal. The upper and lower limits ofpreferred rhenium co-promoter concentrations can be suitably varieddepending upon the particular promoting effect desired or othervariables involved. A possible lower limit of rhenium co-promoter is,for example, about 0.01 micromoles/gram of total catalyst, and apossible upper limit of rhenium co-promoter is, for example, about 16micromoles/gram of total catalyst, measured as the metal.

The amount of nickel deposited, post-impregnated or post-doped on theimpregnated support or catalyst or present on the catalyst is to be apromoting amount. Preferably, the amount of nickel will range from about10 parts per million to about 1000 parts per million, preferably fromabout 15 parts per million to about 500 parts per million, and morepreferably, from about 30 parts per million to about 250 parts permillion by weight of the total catalyst, expressed as the element. Theupper and lower limits of preferred nickel concentrations can besuitably varied depending upon the particular promoting effect desiredor other variables involved. A possible lower limit of nickel is, forexample, about I part per million by weight of the total catalyst,expressed as the element, and a possible upper limit of nickel is, forexample, about 2,000 parts per million by weight of the total catalyst,expressed as the element.

It is observed that independent of the form in which the silver ispresent in the solution before precipitation on the carrier, the term"reduction to metallic silver" is used, while in the meantime oftendecomposition by heating occurs. We prefer to use the term "reduction",since Ag⁺ ion is converted into a metallic Ag atom. Reduction times maygenerally vary from about 0.5 minute to about 8 hours, depending on thecircumstances.

The silver catalysts according to the present invention have been shownto have a particularly high selectivity stability for ethylene oxideproduction in the direct oxidation of ethylene with molecular oxygen toethylene oxide. The conditions for carrying out such an oxidationreaction in the presence of the silver catalysts according to thepresent invention broadly comprise those already described in the priorart. This applies, for example, to suitable temperatures, pressures,residence times, diluent materials such as nitrogen, carbon dioxide,steam, argon, methane or other saturated hydrocarbons, to the presenceof moderating agents to control the catalytic action, for example,1-2-dichloroethane, vinyl chloride, ethyl chloride or chlorinatedpolyphenyl compounds, to the desirability of employing recycleoperations or applying successive conversations in different reactors toincrease the yields of ethylene oxide, and to any other specialconditions which may be selected in processes for preparing ethyleneoxide. Pressures in the range of from atmospheric to about 500 psig aregenerally employed. Higher pressures, however, are not excluded.Molecular oxygen employed as reactant can be obtained from conventionalsources. The suitable oxygen charge may consist essentially orrelatively pure oxygen, a concentrated oxygen stream comprising oxygenin major amount with lesser amounts of one or more diluents, such asnitrogen and argon, or another oxygen-containing stream, such as air. Itis therefore evident that the use of the present silver catalysts inethylene oxide reactions is in no way limited to the use of specificconditions among those which are known to be effective. For purposes ofillustration only, the following table shows the range of conditionsthat are often used in current commercial ethylene oxide reactor units.

                  TABLE II                                                        ______________________________________                                        *GHSV                 1500-10,000                                             Inlet Pressure        150-400 psig                                            Inlet Feed                                                                    Ethylene              1-40%                                                   O.sub.2               3-12%                                                   Ethane                0-3%                                                    Argon and/or methane and/or nitrogen                                                                Balance                                                 diluent                                                                       Chlorohydrocarbon Moderator                                                                         0.3-50 ppmv total                                       Coolant temperature   180-315° C.                                      Catalyst temperature  180-325° C.                                      O.sub.2 conversion level                                                                            10-60%                                                  EO Production (Work Rate)                                                                           2-16 lbs. EO/cu. ft.                                                          catalyst/hr.                                            ______________________________________                                         *Cubic feet of gas at standard temperature and pressure passing over one      cubic foot of packed catalyst per hour.                                  

In a preferred application of the silver catalysts according to thepresent invention, ethylene oxide is produced when an oxygen-containinggas is contacted with ethylene in the presence of the present catalystsat a temperature in the range of from about 180° C. to about 330° C. andpreferably about 200° C. to about 325° C.

The ranges and limitations provided in the instant specification andclaims are those which are believed to particularly point out anddistinctly claim the present invention. It is, however, understood thatother ranges and limitations which perform substantially the samefunction in the same or substantially the same manner to obtain the sameor substantially the same result are intended to be within the scope ofthe instant invention as defined by the instant specification andclaims.

The invention will be illustrated by the following illustrativeembodiments which are provided for illustration only and are notintended to limit the scope of the instant invention.

ILLUSTRATIVE EMBODIMENTS

Illustrative Embodiment 1

The following illustrative embodiment describes typical preparativetechniques for making the catalysts of the instant invention (andcomparative catalysts) and the typical technique for measuring theproperties of these catalysts.

Part A: Preparation of stock silver oxalate/ethylenediamine solution foruse in catalyst preparation:

1) Dissolve 415 grams (g) of reagent-grade sodium hydroxide in 2340milliliters (ml) deionized water. Adjust the temperature to 50° C.

2) Dissolve 1699 g of "Spectropure" (high purity) silver nitrate in 2100ml deionized water. Adjust the temperature to 50° C.

3) Add sodium hydroxide solution slowly to silver nitrate solution withstirring while maintaining a temperature of 50° C. Stir for 15 minutesafter addition is complete, and then lower the temperature to 40° C.

4) Insert clean filter wands and withdraw as much water as possible fromthe precipitate created in step (3) in order to remove sodium andnitrate ions. Measure the conductivity of the water removed and add backas much fresh deionized water as was removed by the filter wands. Stirfor 15 minutes at 40° C. Repeat this process until the conductivity ofthe water removed is less than 90 μmho/cm. Then add back 1500 mldeionized water.

5) Add 630 g of high-purity oxalic acid dihydrate in approximately 100 gincrements. Keep the temperature at 40° C. and stir to mix thoroughly.Add the last portion of oxalic acid dihydrate slowly and monitor pH toensure that pH does not drop below 7.8.

6) Remove as much water from the mixture as possible using clean filterwands in order to form a highly concentrated silver-containing slurry.Cool the silver oxalate slurry to 30° C.

7) Add 699 g of 92 percent weight (% w) ethylenediamine (8% deionizedwater). Do not allow the temperature to exceed 30° C. during addition.

The above procedure yields a solution containing approximately 27-33% wsilver.

Part B: Preparation of impregnated catalysts

For preparing the impregnated catalyst, into a 10 ml beaker is added0.166 g of (NH₄)ReO₄ and approximately 2.0 g of ethylenediamine/H₂ O(50/50 by weight), and the mixture is allowed to dissolve with stirring.0.079 g of Li₂ SO₄.H₂ O is dissolved in 1 ml of water in a weighingdish, and then added to the perrhenate solution. 0.3416 g of LiNO₃ isdissolved in 2 ml of water and added to the perrhenate solution. Theperrhenate/lithium sulfate/lithium nitrate solution is allowed to stir,ensuring complete dissolution. This dopant solution is then added to 184g of the above-prepared silver solution (specific gravity=1.556 g/cc),and the resulting solution is diluted with water to a total weight of204 g. One-fourth of this solution is used to prepare a catalyst.0.0444-0.0475 g of CsOH is added to a 51 g portion of the silveroxalate/dopant solution to prepare the final impregnation solution.

The final impregnation solution thus prepared is then used to impregnatea catalyst carrier in the manner described below. Catalyst carrier Cwhich is described in Table 1 is a preferred support for the instantinvention and is used in the following examples and illustrativeembodiments unless otherwise stated.

Approximately 30 g of carrier C are placed under 25 mm vacuum for 3minutes at room temperature. Approximately 50 g of doped impregnatingsolution is then introduced to submerge the carrier, and the vacuum ismaintained at 25 mm for an additional 3 minutes. At the end of thistime, the vacuum is released, and excess impregnating solution isremoved from the carrier by centrifugation for 2 minutes at 500 rpm. Ifthe impregnating solution is prepared without monoethanolamine, then theimpregnated carrier is then cured by being continuously shaken in a 300cu. ft/hr. air stream flowing across a cross-sectional area ofapproximately 3-5 square inches at 250°-270° C. for 5-6 minutes. Ifsignificant monoethanolamine is present in the impregnating solution,then the impregnated carrier is cured by being continuously shaken in a300 cu. ft./hr. air stream at 250° C. for 2.5 minutes, followed by a 100cu. ft./hr. air stream at 270° C. for 7.5 minutes (all over across-section area of approximately 3-5 square inches). The curednickel-free catalyst (Catalyst C) is then ready for testing.

Part C: Catalyst post-doping procedure

A standard solution of nickel ions is prepared using the followingprocedure. Into a 50 milliliter (ml) volumetric flask is added 12.2911 gof nickel nitrate hexahydrate, 31.06 g of a 50/50 mixture ofethylenediamine and water, and approximately 2.5 milliliters ofconcentrated aqueous ammonium hydroxide. 10 milliliters of saturatedammonium carbonate solution is also added. The solution is then allowedto stand overnight and is then diluted with water to 50 milliliters. Thesolution prepared contains 0.0491 grams of nickel per milliliter ofsolution.

For Catalyst A, in order to deposit 88 parts per million (ppm) of nickelions, the following solution is made. 138.9 microliters (0.1389 ml) ofthe above standard solution of nickel ions is added to 30 milliliters ofabsolute (100%) ethanol. 36.1 grams of Catalyst C. (as prepared above)is then impregnated at 25-50 millimeters (mm) vacuum for three minutes.At the end of this time, the vacuum is released and the excess solutionis decanted from the catalyst pellets. The catalyst pellets are thendried by continuous shaking in a 300 cu. ft./hr. air stream at 60°-80°C., three minutes at 120°-150° C., and four minutes at 250° C.

For Catalyst B, the post-doping procedure for Catalyst A above wascarried out, except that no nickel solution was added to the absolute(100%) ethanol. Catalyst B was thus post-doped with ethanol only.

The procedures set forth above for Catalysts A, B and C will yieldcatalysts on this carrier which contain approximately 13.5% w Ag withthe following approximate dopant levels (expressed in parts per millionby weight basis the weight of the total catalyst, i.e., ppmw) and whichare approximately optimum in cesium for the given silver and rhenium andsulfur levels and support with regard to initial selectivity under thetest conditions described below.

    ______________________________________                                               Cs, ppmw                                                                              Nickel, ppmw                                                                             Re, ppmw  S, ppmw                                   ______________________________________                                        Catalyst A                                                                             460       88         280     48                                      Catalyst B                                                                             460       None       280     48                                      Catalyst C                                                                             430       None       280     48                                      ______________________________________                                    

The actual silver content of the catalyst can be determined by any of anumber of standard, published procedures. The actual level of rhenium onthe catalysts prepared by the above process can be determined byextraction with 20 mM aqueous sodium hydroxide, followed byspectrophotometric determination of the rhenium in the extract. Theactual level of nickel on the catalyst can be determined by standardatomic emission spectroscopy. The actual level of cesium on the catalystcan be determined by employing a stock cesium hydroxide solution, whichhas been labeled with a radioactive isotope of cesium, in catalystpreparation. The cesium content of the catalyst can then be determinedby measuring the radioactivity of the catalyst. Alternatively, thecesium content of the catalyst can be determined by leaching thecatalyst with boiling deionized water. In this extraction processcesium, as well as other alkali metals, is measured by extraction fromthe catalyst by boiling 10 grams of whole catalyst in 20 milliliters ofwater for 5 minutes, repeating the above two more times, combining theabove extractions and determining the amount of alkali metal present bycomparison to standard solutions of reference alkali metals using atomicabsorption spectroscopy (using Varian Techtron Model 1200 orequivalent). It should be noted that the cesium content of the catalystas determined by the water leaching technique may be lower than thecesium content of the catalyst as determined by the radiotracertechnique.

Part D: Standard Microreactor Catalyst Test Conditions/Procedure

3 to 5 grams of crushed catalyst (14-20 mesh) are loaded into a 1/4 inchdiameter stainless steel U-shaped tube. The U tube is immersed in amolten metal bath (heat medium) and the ends are connected to a gas flowsystem. The weight of the catalyst used and the inlet gas flow rate areadjusted to achieve a gas hourly space velocity of 3300 cc of gas per ccof catalyst per hour. The inlet gas pressure is 210 psig.

The gas mixture passed thorough the catalyst bed (in once-throughoperation) during the entire test run (including startup) consists of30% ethylene, 8.5% oxygen, 5% carbon dioxide, 54.5% nitrogen, and 2.0 to6.0 ppmv ethyl chloride.

The initial reactor (heat medium) temperature is 225° C. After 1 hour atthis initial temperature, the temperature is increased to 235° C. for 1hour, and then adjusted to 245° C. for 1 hour. The temperature is thenadjusted so as to achieve a constant oxygen conversion level of 40%.Performance data at this conversion level are usually obtained when thecatalyst has been onstream for a total of at least 2-3 days. Due toslight differences in feed gas composition, gas flow rates, and thecalibration of analytical instruments used to determine the feed andproduct gas compositions, the measured selectivity and activity of agiven catalyst may vary slightly from one test run to the next. To allowmeaningful comparison of the performance of catalysts tested atdifferent times, all catalysts described in this illustrative embodimentwere tested simultaneously with a reference catalyst. All performancedata reported in this illustrative embodiment are corrected relative tothe average initial performance of the reference catalyst (S₄₀ =81.0%;T₄₀ =230° C.).

After obtaining initial performance values for selectivity at 40%conversion the catalysts are subjected to high severity agingconditions. Under these conditions, the catalyst is brought to either85% conversion or a maximum of 285° C. for a 10-day period to acceleratethe aging of the catalyst. After this 10-day aging period, the catalystsare again brought to 40% conversion and reoptimized under standardconditions. The selectivity is again measured, and compared to theoriginal value of the fresh catalyst. After the new selectivity value isdetermined, this cycle is repeated, and the selectivity decline of thecatalyst is continuously measured under standard 40% conversionconditions in 10-day cycles relative to its fresh initial performance.The results are presented below in Table III. All selectivity values areexpressed as %. Catalysts A and C have initial S₄₀ values of 86.0-86.5%at T₄₀ values of 259° C.±4° C. Catalyst B showed an initial S₄₀ valuesof approximately 84%. Catalyst B (post-doped with ethanol) was nottested in a long term run due to its poor initial performance relativeto Catalyst A (post-doped with nickel) and Catalyst C (standardimpregnated with no post-doped nickel).

    Loss of Selectivity (%)={S.sub.40, % (Aged)}-{S.sub.40, % (Fresh)}

                  TABLE III                                                       ______________________________________                                        Loss of Selectivity from Fresh Catalyst                                       Total Days at Either 85% Conversion or 285° C.                         (Data Obtained at 40% Conversion Conditions)                                  Catalyst                                                                              10 Days  20 Days  30 Days                                                                              40 Days                                                                              50 Days                               ______________________________________                                        A (Re/Ni)                                                                             -0.1%    -0.1%    -0.8%  -2.4%  -4.3%                                 C (Re)  -1.2%    -2.3%    -3.4%  -5.0%  -6.1%                                 ______________________________________                                    

As mentioned previously, selectivity decline is of tremendous economicimportance when choosing a catalyst, and retarding this decline rate canlead to significant savings in costs. As can be seen from Table III,Catalyst C, which does not contain a post-doped, i.e., post-impregnated,nickel promoter in combination with Re, decreases in selectivity muchmore rapidly than do the catalysts which contain a post-doped nickelpromoter. Catalysts which contain post-doped nickel promoters incombination with rhenium maintain their selectivity significantly longerthan catalysts without post-doped nickel promoters and are thussignificantly advantaged. Catalyst B's poor initial performance showsthat the advantages which are seen with Catalyst A are not due to thepresence of the ethanol or the post-doping step, but rather due to theaddition of the nickel during the post-doping or post-impregnation step.

What is claimed is:
 1. A process for preparing an ethylene oxidecatalyst for the vapor phase production of ethylene oxide from ethyleneand oxygen which process comprises depositing a catalytically effectiveamount of silver, a promoting amount of alkali metal and a promotingamount of rhenium on a support having a surface area in the range offrom about 0.05 m² /g to about 10 m² /g, at least partially drying thesupport, depositing a promoting amount of nickel on the support, andthereafter drying the support.
 2. The process of claim 1 wherein thesupport comprises a porous refractory oxide.
 3. The process of claim 2wherein the support comprises alpha alumina.
 4. The process of claim 3wherein the support surface area is in the range of from about 0.05 m²/g to about 5 m² /g.
 5. The process of claim 1 wherein the amount ofsilver is in the range of from about 1 percent by weight to about 40percent by weight of the total catalyst, the amount of alkali metalpromoter is in the range of from about 10 parts per million to about1500 parts per million, expressed as the metal, by weight of the totalcatalyst, the amount of rhenium promoter is in the range of from about0.1 micromoles to about 10 micromoles of rhenium, expressed as themetal, per gram of total catalyst, and the amount of nickel promoter isin the range of from about 10 parts per million to about 1000 parts permillion, expressed as the element, by weight of the total catalyst. 6.The process of claim 1 wherein the alkali metal, rhenium and nickel arefound individually or in any mixture thereof on the catalyst, on thesupport or on both the catalyst and the support.
 7. The process of claim1 wherein the alkali metal is selected from potassium, rubidium, cesium,and mixtures thereof.
 8. The process of claim 7 wherein the alkali metalis potassium.
 9. The process of claim 7 wherein the alkali metal isrubidium.
 10. The process of claim 7 wherein the alkali metal is cesium.11. The process of claim 7 wherein the alkali metal comprises cesiumplus at least one additional alkali metal.
 12. The process of claim 11wherein the alkali metal is cesium plus potassium.
 13. The process ofclaim 11 wherein the alkali metal is cesium plus rubidium.
 14. Theprocess of claim 11 wherein the alkali metal is cesium plus lithium. 15.The process of claim 1 wherein the nickel is applied to the support as asolubilized nickel compound.
 16. The process of claim 15 wherein thenickel compound is selected from nickel carbonate, nickel nitrate,nickel acetate, nickel acetylacetonate, nickel chloride, nickel sulfateand mixtures thereof.
 17. The process of claim 15 wherein the nickelcompound is selected from nickel carbonate, nickel nitrate, nickelsulfate and mixtures thereof.
 18. The process of claim 15 wherein thenickel compound is nickel carbonate.
 19. The process of claim 15 whereinthe nickel compound is nickel nitrate.
 20. The process of claim 1wherein the rhenium is applied to the support in the form of aperrhenate or rhenium heptoxide.
 21. A process for preparing a catalystfor the vapor phase production of ethylene oxide from ethylene andmolecular oxygen which comprises depositing silver, an alkali metalpromoter, and rhenium promoter on a porous refractory support having asurface area in the range of from about 0.05 m² /g to about 10 m² /g, atleast partially drying the support, depositing on the support from about10 parts per million to about 1000 parts per million by weight of thetotal catalyst of nickel promoter, expressed as the element, wherein thenickel is applied to the support as a solubilized nickel compound, andthereafter drying the support.
 22. A process for preparing an ethyleneoxide catalyst for the vapor phase production of ethylene oxide fromethylene and oxygen which comprises depositing a catalytically effectiveamount of silver, a promoting amount of alkali metal, a promoting amountof rhenium and a rhenium co-promoter selected from sulfur, molybdenum,tungsten, chromium and mixtures thereof, on a suitable support having asurface area in the range of from about 0.05 m² /g to about 10 m² /g, atleast partially drying the support, depositing a promoting amount ofnickel on the support, and thereafter drying the support.
 23. Theprocess of claim 22 wherein said support comprises a porous refractoryoxide.
 24. The process of claim 23 wherein the support comprises alphaalumina.
 25. The process of claim 24 wherein the support surface area isin the range of from about 0.05 m² /g to about 5 m² /g.
 26. The processof claim 22 wherein the amount of silver is in the range of from about 1percent by weight to about 40 percent by weight of the total catalyst,the amount of alkali metal promoter is in the range of from about 10parts per million to about 3000 parts per million, expressed as themetal, by weight of the total catalyst, the amount of rhenium promoteris in the range of from about 0.1 micromoles to about 10 micromoles ofrhenium, expressed as the metal, per gram of total catalyst and theamount of rhenium co-promoter is in the range of from about 0.1micromoles to about 10 micromoles of rhenium, expressed as the element,per gram of total catalyst, and the amount of nickel promoter is in therange of from about 10 parts per million to about 1000 parts permillion, expressed as the element, by weight of the total catalyst. 27.The process of claim 22 wherein the rhenium and rhenium co-promoter arefound individually or in any mixture thereof on the surface of thesupport or on the surface of the catalyst.
 28. The process of claim 22wherein the alkali metal, rhenium, rhenium co-promoter and nickel arefound individually or in any mixture thereof on the catalyst, on thesupport or on both the catalyst and the support.
 29. The process ofclaim 22 wherein the alkali metal is selected from potassium, rubidium,cesium, and mixtures thereof.
 30. The process of claim 29 wherein thealkali metal is potassium.
 31. The process of claim 29 wherein thealkali metal is rubidium.
 32. The process of claim 29 wherein the alkalimetal is cesium.
 33. The process of claim 29 wherein the alkali metal iscesium plus at least one additional alkali metal.
 34. The process ofclaim 33 wherein the alkali metal is cesium plus potassium.
 35. Theprocess of claim 33 wherein the alkali metal is cesium plus rubidium.36. The process of claim 33 wherein the alkali metal is cesium pluslithium.
 37. The process of claim 22 wherein the nickel is applied tothe support as a solubilized nickel compound.
 38. The process of claim37 wherein the nickel compound is selected from nickel carbonate, nickelnitrate, nickel acetate, nickel acetylacetonate, nickel chloride, nickelsulfate and mixtures thereof.
 39. The process of claim 37 wherein thenickel compound is selected from nickel carbonate, nickel nitrate,nickel sulfate and mixtures thereof.
 40. The process of claim 37 whereinthe nickel compound is nickel carbonate.
 41. The process of claim 37wherein the nickel compound is nickel nitrate.
 42. The process of claim22 wherein the rhenium is applied to the support in the form of aperrhenate or rhenium heptoxide.
 43. The process of claim 22 wherein therhenium co-promoter comprises an oxidic compound or an oxyanion.
 44. Theprocess of claim 22 wherein the rhenium co-promoter is selected fromsulfate, sulfite, sulfonate, molybdate, tungstate, chromate and mixturesthereof.
 45. A process preparing a catalyst for the vapor phaseproduction of ethylene oxide from ethylene and molecular oxygen whichcomprises depositing silver, an alkali metal promoter, a rheniumpromoter and a rhenium co-promoter selected from sulfur, molybdenum,tungsten, chromium and mixtures thereof, on a porous refractory supporthaving a surface area in the range of from about 0.05 m² /g to about 10m² /g, at least partially drying the support, depositing on the supportfrom about 10 parts per million to about 1000 parts per million ofnickel promoter, expressed as the element, and thereafter drying thesupport, wherein the nickel is applied to the support as a solubilizednickel compound.
 46. A process for preparing a catalyst for the vaporphase production of ethylene oxide from ethylene and molecular oxygenwhich process comprises impregnating a porous refractory oxide with acatalytically effective amount of silver, a promoting amount of alkalimetal and a promoting amount of rhenium, at least partially drying theimpregnated porous refractory oxide, post-impregnating the impregnatedporous refractory oxide with a promoting amount of nickel, and dryingthe post-impregnated porous refractory oxide.
 47. The process of claim46 wherein the alkali metal is selected from potassium, rubidium,cesium, lithium, sodium and mixtures thereof.
 48. The process of claim47 wherein the alkali metal is cesium plus at least one additionalalkali metal.
 49. The process of claim 46 wherein the nickel is appliedto the support as a solubilized nickel compound.
 50. The process ofclaim 49 wherein the nickel compound is selected from nickel carbonate,nickel nitrate, nickel acetate, nickel acetylacetonate, nickel chloride,nickel sulfate and mixtures thereof.
 51. The process of claim 49 whereinthe nickel compound is selected from nickel carbonate, nickel nitrate,nickel sulfate and mixtures thereof.
 52. The process of claim 49 whereinthe nickel compound is nickel carbonate.
 53. The process of claim 49wherein the nickel compound is nickel nitrate.
 54. The process of claim46 wherein the rhenium is applied to the support in the form of aperrhenate or rhenium heptoxide.
 55. A process for preparing a catalystfor the vapor phase production of ethylene oxide from ethylene andmolecular oxygen which comprises impregnating a porous refractorysupport with one or more solutions comprising silver, alkali metal andrhenium, wherein the concentration of the silver (expressed as themetal) in the solution ranges from about 1 gram/liter to the solubilitylimit of silver in the solution, the concentration of alkali metal(expressed as the metal) in the solution ranges from about 1×10⁻³grams/liter to about 12 grams/liter and the concentration of the rhenium(expressed as the metal) in the solution ranges from about 5×10⁻³grams/liter to about 20 grams/liter, at least partially drying theimpregnated support, post-impregnating the impregnated support with oneor more solutions comprising nickel, wherein the concentration of thenickel (expressed as the element) in the solution ranges from about1×10⁻⁵ grams/liter to about 1 gram/liter, and thereafter drying thepost-impregnated support, to provide the catalyst with a catalyticallyeffective amount of silver, a promoting amount of alkali metal, apromoting amount of rhenium and a promoting amount of nickel.
 56. Theprocess of claim 55 wherein the solution containing silver alsocomprises water and vicinal alkylenediamine(s) of from 2 to 4 carbonatoms.
 57. The process of claim 55 wherein the solution containingsilver also comprises water, vicinal alkylenediamine(s) of from 2 to 4carbon atoms and vicinal alkanolamine(s) of from 2 to 4 carbon atoms.58. A process for preparing a catalyst for the vapor phase production ofethylene oxide from ethylene and molecular oxygen which comprisesimpregnating a porous refractory support comprising alpha alumina withone or more solutions comprising solvent having silver compound(s)dissolved therein, alkali metal compound(s) dissolved therein andrhenium compound(s) dissolved therein sufficient to deposit on thesupport from about 1 percent by weight to about 40 percent by weight ofthe total catalyst of the silver compound(s), expressed as the metal,from about 10 parts per million to about 3000 parts per million byweight of alkali metal compound(s), expressed as the metal, by weight ofthe total catalyst, and from about 0.1 micromoles to about 10 micromolesper gram of total catalyst of rhenium compound(s), expressed as themetal, at least partially drying the impregnated support,post-impregnating the impregnated support with one or more solutionscomprising solvent having nickel compound(s) dissolved thereinsufficient to deposit on the impregnated support from about 10 parts permillion to about 1000 parts per million by weight of the total catalystof nickel compound(s), expressed as the element, and thereafter dryingthe post-impregnated support to provide the catalyst with acatalytically effective amount of silver, a promoting amount of alkalimetal, a promoting amount of rhenium and a promoting amount of nickel.59. The process of claim 58 wherein the silver compound is selected fromsilver oxalate, silver oxide, silver carbonate, silver lactate andmixtures thereof.
 60. The process of claim 58 wherein the alkali metalcompound is a hydroxide, sulfate and/or nitrate.
 61. The process ofclaim 58 wherein the nickel compound is selected from nickel carbonate,nickel nitrate, nickel acetate, nickel acetylacetonate, nickel chloride,nickel sulfate and mixtures thereof.
 62. The process of claim 58 whereinthe rhenium-containing compound is ammonium and/or alkali metalperrhenate and/or rhenium heptoxide.
 63. A process for preparing acatalyst for the vapor phase production of ethylene oxide from ethyleneand molecular oxygen which process comprises impregnating a porousrefractory oxide with a catalytically effective amount of silver, apromoting amount of alkali metal, a promoting amount of rhenium and arhenium co-promoter selected from sulfur, molybdenum, tungsten, chromiumand mixtures, at least partially drying the impregnated porousrefractory oxide, post-impregnating the impregnated porous refractoryoxide with a promoting amount of nickel, and thereafter drying thepost-impregnated porous refractory oxide.
 64. The process of claim 63wherein the alkali metal is selected from potassium, rubidium, cesium,lithium, sodium and mixtures thereof.
 65. The process of claim 64wherein the alkali metal is cesium plus at least one additional alkalimetal.
 66. The process of claim 63 wherein the nickel is applied to thesupport as a solubilized nickel compound.
 67. The process of claim 66wherein the nickel compound is selected from nickel carbonate, nickelnitrate, nickel acetate, nickel acetylacetonate, nickel chloride, nickelsulfate and mixtures thereof.
 68. The process of claim 66 wherein thenickel compound is selected from nickel carbonate, nickel nitrate,nickel sulfate and mixtures thereof.
 69. The process of claim 66 whereinthe nickel compound is nickel carbonate.
 70. The process of claim 66wherein the nickel compound is nickel nitrate.
 71. The process of claim63 wherein the rhenium is applied to the support in the form of aperrhenate or rhenium heptoxide.
 72. A process for preparing a catalystfor the vapor phase production of ethylene oxide from ethylene andmolecular oxygen which comprises impregnating a porous refractorysupport with one or more solutions comprising silver, alkali metal,rhenium and rhenium co-promoter, wherein the concentration of the silver(expressed as the metal) in the solution ranges from about 1 gram/literto the solubility limit of silver in the solution, the concentration ofalkali metal (expressed as the metal) in the solution ranges from about1×10⁻³ grams/liter to about 12 grams/liter, the concentration of therhenium (expressed as the metal) in the solution ranges from about5×10⁻³ grams/liter to about 20 grams/liter and the concentration of therhenium co-promoter (expressed as the element) in the solution rangesfrom about 1×10⁻³ grams/liter to about 20 grams/liter, partially dryingthe impregnated support, post-impregnating the impregnated support withone or more solutions comprising nickel, wherein the concentration ofthe nickel (expressed as the element) in the solution ranges from about1×10⁻⁵ grams/liter to about 1 gram/liter, and drying thepost-impregnated support, to provide the catalyst with a catalyticallyeffective amount of silver, a promoting amount of alkali metal, apromoting amount rhenium, a promoting amount of rhenium co-promoter anda promoting amount of nickel.
 73. The process of claim 72 wherein thesolution containing silver also comprises water and vicinalalkylenediamine(s) of from 2 to 4 carbon atoms.
 74. The process of claim72 wherein the solution containing silver also comprises water, vicinalalkylenediamine(s) of from 2 to 4 carbon atoms and vicinalalkanolamine(s) of from 2 to 4 carbon atoms.
 75. A process for preparinga catalyst for the vapor phase production of ethylene oxide fromethylene and molecular oxygen which comprising impregnating a porousrefractory support comprising alpha alumina with one or more solutionscomprising solvent having silver compound(s) dissolved therein, alkalimetal compound(s) dissolved therein, and rhenium compound(s) dissolvedtherein sufficient to deposit on the support from about 1 percent byweight to about 40 percent by weight of the total catalyst of the silvercompound(s), expressed as the metal, from about 10 parts per million toabout 3000 parts per million by weight of alkali metal compound(s),expressed as the metal, by weight of the total catalyst, from about 0.1micromoles to about 10 micromoles per gram of total catalyst of rheniumcompound(s), expressed as the metal, and from about 0.1 micromoles toabout 10 micromoles per gram of total catalyst of rhenium co-promotercompound(s), expressed as the element, at least partially drying theimpregnated support, and thereafter post-impregnating the impregnatedsupport with one or more solutions comprising solvent having nickelcompound(s) dissolved therein sufficient to deposit on the impregnatedsupport from about 10 parts per million to about 1000 parts per millionby weight of the total catalyst of nickel compound(s), expressed as theelement, to provide the catalyst with a catalytically effective amountof silver, a promoting amount of alkali metal, a promoting amount ofrhenium, a promoting amount of rhenium co-promoter, and a promotingamount of nickel.
 76. The process of claim 75 wherein the silvercompound is selected from silver oxalate, silver oxide, silvercarbonate, silver lactate and mixtures thereof.
 77. The process of claim75 wherein the alkali metal compound is a hydroxide, sulfate and/ornitrate.
 78. The process of claim 75 wherein the nickel compound isselected from nickel carbonate, nickel nitrate, nickel acetate, nickelacetylacetonate, nickel chloride, nickel sulfate and mixtures thereof.79. The process of claim 75 wherein the nickel compound is selected fromnickel carbonate, nickel nitrate, nickel sulfate and mixtures thereof.80. The process of claim 75 wherein the nickel compound is nickelcarbonate.
 81. The process of claim 75 wherein the nickel compound isnickel nitrate.
 82. The process of claim 75 wherein therhenium-containing compound is ammonium and/or alkali metal perrhenateand/or rhenium heptoxide.
 83. The process of claim 75 wherein therhenium co-promoter compound is selected from compounds of sulfur,tungsten, molybdenum, chromium and mixtures thereof.
 84. The process ofclaim 83 wherein the sulfur compound is selected from ammonium sulfate,alkali metal sulfate and mixtures thereof, the tungsten compound isselected from ammonium tungstate, alkali metal tungstate, tungstic acidand mixtures thereof, the molybdenum compound is selected from ammoniummolybdate, alkali metal molybdate, molybdic acid and mixtures thereof,and the chromium compound is selected from ammonium chromate, alkalimetal chromate, dichromate, chromic acid, dichromic acid and mixturesthereof.